However, the 1st, 2nd and 3rd ionization energies have been measured.

Look for at what point does the ionization energy make a "sudden increase" in magnitude.

The amount of energy required is called the ionization energy.

This distribution is determined by the atomic ionization energy, and the local temperature.

Simply put, the lower the principal quantum number, the higher the ionization energy for the electrons within that shell.

The chemical elements to the left of the periodic table have a much lower ionization energy.

The ones to the right have a much higher ionization energy.

Thus, all of the ionization energy is deposited in an extremely small volume near its original location.

Moving left to right within a period or upward within a group, the first ionization energy generally increases.

This decrease in atomic radius also causes the ionization energy to increase when moving from left to right across a period.